The Hungarian physicist Leó Szilárd, who was living in the United States at the time, realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction that could yield vast amounts of energy for electric power generation or atomic bombs. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Ernest Rutherford's disparaging remarks about generating power from his team's 1932 experiment using protons to split lithium. However, Szilárd had not been able to achieve a neutron-driven chain reaction with neutron-rich light atoms. In theory, if in a neutron-driven chain reaction the number of secondary neutrons produced was greater than one, then each such reaction could trigger multiple additional reactions, producing an exponentially increasing number of reactions.[2]

Szilárd teamed up with Fermi to build a nuclear reactor from natural uranium at Columbia University, where they were fortunate in having a sympathetic head of the physics department in George B. Pegram. At the time there was disagreement about whether it was uranium-235, which made up less than 1% of natural uranium, or the more abundant uranium-238isotope, as Fermi maintained, that was primarily responsible for fission. Fermi and Szilárd conducted a series of experiments, and concluded that a chain reaction in natural uranium could be possible if they could find a suitable neutron moderator. They found that the hydrogen atoms in water slowed neutrons, but tended to capture them. Szilárd then suggested using carbon as a moderator. They now needed large quantities of carbon and uranium to create a reactor. Szilárd was convinced that they would succeed if only they could get the materials.[3]

Szilárd was concerned that German scientists might also attempt this experiment. The German nuclear physicist Siegfried Flügge published two influential articles on the exploitation of nuclear energy in 1939.[4][5] After discussing this prospect with fellow Hungarian physicist Eugene Wigner, they decided that they should warn the Belgians, as the Belgian Congo was the best source of uranium ore. Wigner suggested that Albert Einstein might be a suitable person to do this, as he knew the Belgian Royal Family.[6]

The letter

On July 12, 1939, Szilárd and Wigner drove in Wigner's car to Cutchogue on New York's Long Island, where Einstein was staying.[9] When they explained about the possibility of atomic bombs, Einstein replied: Daran habe ich gar nicht gedacht (I did not even think about that).[10] Szilárd dictated a letter in German to the Belgian Ambassador to the United States. Wigner wrote it down, and Einstein signed it. At Wigner's suggestion, they also prepared a letter for the State Department explaining what they were doing and why, giving it two weeks to respond if it had any objections.[9]

This still left the problem of getting government support for uranium research. Another friend of Szilárd's, the Austrian economist Gustav Stolper, suggested approaching Alexander Sachs, who had access to PresidentFranklin D. Roosevelt. Sachs told Szilárd that he had already spoken to the President about uranium, but that Fermi and Pegram had reported that the prospects for building an atomic bomb were remote. He told Szilárd that he would deliver the letter, but suggested that it come from someone more prestigious. For Szilárd, Einstein was again the obvious choice.[6] Sachs and Szilárd drafted a letter riddled with spelling errors and mailed it to Einstein.[11]

Szilárd set out for Long Island again on August 2. Wigner was unavailable, so this time Szilárd co-opted another Hungarian physicist, Edward Teller, to do the driving. Einstein dictated the letter in German. On returning to Columbia University, Szilárd dictated the letter in English to a young departmental stenographer, Janet Coatesworth. She later recalled that when Szilárd mentioned extremely powerful bombs, she "was sure she was working for a nut".[12] Ending the letter with "Yours truly, Albert Einstein" did nothing to alter this impression. Both the letter and a longer explanatory letter were then posted to Einstein.[12]

The letter warned that:

"In the course of the last four months it has been made probable — through the work of Joliot in France as well as Fermi and Szilárd in America — that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.

This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air."[13]

It also specifically warned about Germany:

"I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated."[13]

At the time of the letter, the estimated material necessary for a fission chain reaction was several tons. Seven months later a breakthrough in Britain would estimate the necessary critical mass to be less than 10 kilograms, making delivery of a bomb by air a possibility.[14]

Delivery

Roosevelt's reply

The Einstein–Szilárd letter was signed by Einstein and posted back to Szilárd, who received it on August 9.[12] Szilárd gave both the short and long letters, along with a letter of his own, to Sachs on August 15. Sachs asked the White House staff for an appointment to see President Roosevelt, but before one could be set up, the administration became embroiled in a crisis due to Germany's invasion of Poland, which started World War II.[15] Sachs delayed his appointment until October so that the President would give the letter due attention, securing an appointment on October 11. On that date he met with the President, the President's secretary, Brigadier GeneralEdwin "Pa" Watson, and two ordnance experts, Army Lieutenant Colonel Keith F. Adamson and Navy Commander Gilbert C. Hoover. Roosevelt summed up the conversation as: "Alex, what you are after is to see that the Nazis don't blow us up."[16]

Roosevelt sent a reply thanking Einstein, and informing him that:

"I found this data of such import that I have convened a Board consisting of the head of the Bureau of Standards and a chosen representative of the Army and Navy to thoroughly investigate the possibilities of your suggestion regarding the element of uranium."[17]

Einstein sent two more letters to Roosevelt, on March 7, 1940, and April 25, 1940, calling for action on nuclear research. Szilárd drafted a fourth letter for Einstein's signature that urged the President to meet with Szilárd to discuss policy on nuclear energy. Dated March 25, 1945, it did not reach Roosevelt before his death on April 12, 1945.[13]

Results

Roosevelt decided that the letter required action, and authorized the creation of the Advisory Committee on Uranium. The committee was chaired by Lyman James Briggs, the Director of the Bureau of Standards (currently the National Institute of Standards and Technology), with Adamson and Hoover as its other members. It convened for the first time on October 21. The meeting was also attended by Fred L. Mohler from the Bureau of Standards, Richard B. Roberts of the Carnegie Institution of Washington, and Szilárd, Teller and Wigner. Adamson was skeptical about the prospect of building an atomic bomb, but was willing to authorize $6,000 ($100,000 in current USD) for the purchase of uranium and graphite for Szilárd and Fermi's experiment.[18]

Einstein did not work on the Manhattan Project. The Army denied him the work clearance needed in July 1940, saying his pacifist leanings made him a security risk,[23] although he was allowed to work as a consultant to the United States Navy's Bureau of Ordnance.[24][25] He had no knowledge of the atomic bomb's development, and no influence on the decision for the bomb to be dropped.[13][23] According to Linus Pauling, Einstein later regretted signing the letter because it led to the development and use of the atomic bomb in combat, adding that Einstein had justified his decision because of the greater danger that Nazi Germany would develop the bomb first.[26] In 1947 Einstein told Newsweek magazine that "had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing."[23][27]

was a common year starting on Sunday of the Gregorian calendar, the 1939th year of the Common Era (CE) and Anno Domini (AD) designations, the 939th year of the 2nd millennium, the 39th year of the 20th century, and the 10th and last year of the 1930s decade. This year also marks the start of the Second World War, the largest and deadliest conflict in human history.

Alexander Sachs (August 1, 1893 – June 23, 1973) was an economist and banker. In October 1939 he delivered the Einstein–Szilárd letter to President Franklin D. Roosevelt, suggesting that nuclear-fission research ought to be pursued with a view to possibly constructing nuclear weapons, should they prove feasible, in view of the likelihood that Nazi Germany would do so. This led to the initiation of the United States' Manhattan Project.

"Atomic spies" or "atom spies" were people in the United States, Great Britain, and Canada who are known to have illicitly given information about nuclear weapons production or design to the Soviet Union during World War II and the early Cold War. Exactly what was given, and whether everyone on the list gave it are still matters of some scholarly dispute, and in some cases, what were originally seen as strong testimonies or confessions were admitted as fabricated in later years. Their work constitutes the most publicly well-known and well-documented case of nuclear espionage in the history of nuclear weapons. There was a movement among nuclear scientists to share the information with the world scientific community, but it was firmly quashed by the U.S. government.

Confirmation about espionage work came from the Venona project, which intercepted and decrypted Soviet intelligence reports sent during and after World War II. They provided clues to the identity of several spies at Los Alamos and elsewhere, some of whom have never been identified. Some of this information was available, but not usable in court for secrecy reasons during the 1950s trials. Additionally, records from Soviet archives, which were briefly opened to researchers after the fall of the Soviet Union, included more information about some spies.

Leo Szilard (; Hungarian: Szilárd Leó [ˈsilaːrd ˈlɛoː]; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-German-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear reactor with Enrico Fermi in 1934, and in late 1939 wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb.

Szilard initially attended Palatine Joseph Technical University in Budapest, but his engineering studies were interrupted by service in the Austro-Hungarian Army during World War I. He left Hungary for Germany in 1919, enrolling at Technische Hochschule (Institute of Technology) in Berlin-Charlottenburg, but became bored with engineering and transferred to Friedrich Wilhelm University, where he studied physics. He wrote his doctoral thesis on Maxwell's demon, a long-standing puzzle in the philosophy of thermal and statistical physics. Szilard was the first to recognize the connection between thermodynamics and information theory.

In addition to the nuclear reactor, Szilard submitted patent applications for a linear accelerator in 1928, and a cyclotron in 1929. He also conceived the idea of an electron microscope. Between 1926 and 1930, he worked with Einstein on the development of the Einstein refrigerator. After Adolf Hitler became chancellor of Germany in 1933, Szilard urged his family and friends to flee Europe while they still could. He moved to England, where he helped found the Academic Assistance Council, an organization dedicated to helping refugee scholars find new jobs. While in England he discovered a means of isotope separation known as the Szilard–Chalmers effect.

Foreseeing another war in Europe, Szilard moved to the United States in 1938, where he worked with Enrico Fermi and Walter Zinn on means of creating a nuclear chain reaction. He was present when this was achieved on December 2, 1942. He worked for the Manhattan Project's Metallurgical Laboratory on aspects of nuclear reactor design. He drafted the Szilard petition advocating a demonstration of the atomic bomb, but the Interim Committee chose to use them against cities without warning.

After the war, Szilard switched to biology. He invented the chemostat, discovered feedback inhibition, and was involved in the first cloning of a human cell. He publicly sounded the alarm against the possible development of salted thermonuclear bombs, a new kind of nuclear weapon that might annihilate mankind. Diagnosed with bladder cancer in 1960, he underwent a cobalt-60 treatment that he had designed. He helped found the Salk Institute for Biological Studies, where he became a resident fellow. Szilard founded Council for a Livable World in 1962 to deliver "the sweet voice of reason" about nuclear weapons to Congress, the White House, and the American public. He died in his sleep of a heart attack in 1964. According to György Marx he was one of The Martians.

Lyman James Briggs (May 7, 1874 – March 25, 1963) was an American engineer, physicist and administrator. He was a director of the National Bureau of Standards during the Great Depression and chairman of the Uranium Committee before America entered the Second World War. The Lyman Briggs College at Michigan State University is named in his honor.

Jewish Americans have served in the United States armed forces dating back to before the colonial era, when Jews had served in militias of the Thirteen Colonies. Jewish military personnel have served in all branches of the armed forces and in every major armed conflict to which the United States has been involved. The Jewish Chaplains Council estimated that there are currently 10,000 known Jewish servicemen and servicewomen on active duty.A number of Jewish American servicemen have gained fame due to their military service, and many have received awards and decorations for distinguished service, valor, or heroism. More than 20 Jewish servicemen were awarded the military's highest award, the Medal of Honor. Many other American Jews who served in the military later achieved prominence in business, politics, science, entertainment and other fields. Foreign Jews have also been significant in the development of military science and technology – including physicists Albert Einstein, J. Robert Oppenheimer, Richard Feynman, and Edward Teller, who were important in the Manhattan Project, which led to the development of the first nuclear weapons.

The National Defense Research Committee (NDRC) was an organization created "to coordinate, supervise, and conduct scientific research on the problems underlying the development, production, and use of mechanisms and devices of warfare" in the United States from June 27, 1940, until June 28, 1941. Most of its work was done with the strictest secrecy, and it began research of what would become some of the most important technology during World War II, including radar and the atomic bomb. It was superseded by the Office of Scientific Research and Development in 1941, and reduced to merely an advisory organization until it was eventually terminated during 1947.

A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid (water or gas), which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research. As of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world.

The Uranium Committee was a committee of the National Defense Research Committee (NDRC) that succeeded the Advisory Committee on Uranium and later evolved into the S-1 Section of the Office of Scientific Research and Development (OSRD), when that organization absorbed the NDRC in June 1941, and the S-1 Executive Committee in June 1942. It laid the groundwork for the Manhattan Project by initiating and coordinating the early research efforts in the United States, and liaising with the Tube Alloys Project in Britain.

The Szilárd petition, drafted by scientist Leo Szilard, was signed by 70 scientists working on the Manhattan Project in Oak Ridge, Tennessee, and the Metallurgical Laboratory in Chicago, Illinois. It was circulated in July 1945 and asked President Harry S. Truman to inform Japan of the terms of surrender demanded by the allies, and allow Japan to either accept or refuse these terms, before America used atomic weapons. However, the petition never made it through the chain of command to President Truman. It also was not declassified and made public until 1961.

Later, in 1946, Szilard jointly with Albert Einstein, created the Emergency Committee of Atomic Scientists that counted among its board, Linus Pauling (Nobel Peace Prize in 1962).

This timeline of nuclear weapons development is a chronological catalog of the evolution of nuclear weapons rooting from the development of the science surrounding nuclear fission and nuclear fusion. In addition to the scientific advancements, this timeline also includes several political events relating to the development of nuclear weapons. The availability of intelligence on recent advancements in nuclear weapons of several major countries (such as United States and the Soviet Union) is limited because of the classification of technical knowledge of nuclear weapons development.

The Manhattan Project was a research and development project that produced the first atomic bombs during World War II. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army Corps of Engineers. The Army component of the project was designated the Manhattan District; "Manhattan" gradually became the codename for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion (about $28 billion in 2018 dollars). Over 90% of the cost was for building factories and producing the fissionable materials, with less than 10% for development and production of the weapons.Two types of atomic bombs were developed during the war. A relatively simple gun-type fission weapon was made using uranium-235, an isotope that makes up only 0.7 percent of natural uranium. Since it is chemically identical to the most common isotope, uranium-238, and has almost the same mass, it proved difficult to separate. Three methods were employed for uranium enrichment: electromagnetic, gaseous and thermal. Most of this work was performed at Oak Ridge, Tennessee. In parallel with the work on uranium was an effort to produce plutonium. Reactors were constructed at Oak Ridge and Hanford, Washington, in which uranium was irradiated and transmuted into plutonium. The plutonium was then chemically separated from the uranium. The gun-type design proved impractical to use with plutonium so a more complex implosion-type nuclear weapon was developed in a concerted design and construction effort at the project's principal research and design laboratory in Los Alamos, New Mexico.

The following is a timeline of the Manhattan Project. It includes a number of events prior to the official formation of the Manhattan Project, and a number of events after the atomic bombings of Hiroshima and Nagasaki, until the Manhattan Project was formally replaced by the Atomic Energy Commission in 1947.

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